What is the ruling on someone who smiles upon hearing or seeing something related to religion, or upon imagining disbelief, to the point where they experience recurring doubt, perform ghusl, and recite the Shahada due to their belief in having committed an act of disbelief? Is such a person considered a disbeliever due to that smile?

The feeling of doubt about falling into disbelief (kufr) does not mean apostasy. The fundamental principle is that whoever enters Islam with certainty does not exit it except with certainty. Scholars have stated that if there are 99 reasons to declare a Muslim a disbeliever and one reason to keep him within Islam, then the latter reason should be acted upon. Apostasy occurs by severing one's Islam through an intention, a statement, or an act of disbelief, whether it be mockery, obstinacy, or conviction. It also occurs by denying the Creator or the Messengers, or by disbelieving a Messenger, or by permitting something unanimously forbidden, or by denying an obligation that is unanimously agreed upon, or by intending disbelief, or by doubting it. Acts of disbelief include explicit mockery of the religion or its denial, such as throwing the Qur'an into filth or prostrating to an idol. Disbelief occurs by word (such as reviling Allah or the Messenger), or by action (such as prostrating to an idol), or by belief (such as believing that adultery is permissible), or by doubting any of these. The remedy for whispers of disbelief is to ignore them and turn to Allah through supplication.